This invention is generally directed to processes for the preparation of toners, and more specifically the present invention is directed to suspension polymerization processes for the preparation of toners. In one embodiment of the present invention, the process comprises the aqueous phase suspension free radical polymerization for the formation of toner polymer resins, such as styrene butadienes, which process is economical in that, for example, it can be accomplished in a rapid time period as compared to prior art processes as illustrated, for example, in U.S. Pat. No. 4,588,108, the disclosure of which is totally incorporated herein by reference. In one embodiment the process of the present invention can be accomplished in a period of time of from about 4.5 hours to about 5.5 hours and preferably in about 4.8 hours, or about 1 hour less than the processes as illustrated in the prior art, reference for example U.S. Pat. No. 4,588,108. Other advantages associated with the processes of the present invention in embodiments thereof include an initiator that allows, for example, an economical time reduced process, and efficient reduction in residual styrene monomer levels. As a consequence, although the reaction time is reduced, residual monomer levels do not increase in embodiments of the present invention. There is also provided in accordance with the present invention toner compositions comprised of resin particles obtained by the processes illustrated herein, pigment particles, and optional additives such as waxes with hydroxyl functionality, charge enhancing components, metal salts, metal salts of fatty acids, colloidal silicas and the like. In addition, the present invention is directed to developer compositions comprised of the aforementioned toners, and carrier particles. Furthermore, in another embodiment of the present invention there are provided single component toner compositions comprised of resin particles obtained by the processes illustrated herein, magnetic components such as magnetites, and optional additives such as waxes with hydroxyl functionality. The toner and developer compositions of the present invention are useful in electrostatographic imaging and printing systems.
A polymer which has been developed to exhibit properties that can satisfy the stringent standards of advanced copier and duplicators is a copolymer of styrene and butadiene comprised of, for example, a certain ratio of styrene to butadiene moieties, a well certain minimum glass transition temperature and a carefully controlled weigh average molecular weight (M.sub.w) range. Emulsion polymerization is a process that can be selected for the preparation of such polymers. However, emulsion polymerization processes have a number of disadvantages including complicated and difficult to control coagulation operations necessary to separate the solid polymer from the latex produced during the emulsion polymerization process. Further, such polymerization processes result in undesirable residual contaminants. In addition, emulsion polymerization techniques can be relatively costly due to the complex processing steps required to form and separate the polymers. Attempts have been made to polymerize copolymers of styrene and butadiene in an aqueous medium to form styrene-butadiene copolymer particles. Examples of these techniques are illustrated in U.S. Pat. Nos. 2,836,584; 4,169,828 and 4,170,699. Unfortunately, these processes have attendant disadvantages when selected for the preparation of toner resins. For example, the process of U.S. Pat. No. 2,836,584 requires polyvinyl alcohol to prevent the formation of a latex, a redox type polymerization initiator or catalyst, and a long chain mercaptan to control the physical properties of the copolymer. Further, materials containing mercaptans emit hydrogen sulfide into the atmosphere and the sulfurous compounds are absorbed by paper substrates during flash fusing due to the decomposition of the mercaptan. The odor of hydrogen sulfide in xerographic copies renders such consumer products unacceptable. The polymerization processes described in U.S. Pat. Nos. 4,169,828 and 4,170,699 require the presence of a bisulfite or persulfate modifier in the presence of adjunct modifier such as amino acids or glutamic acid which adversely affect the electrical properties of electrostatic toners prepared with these materials. The polymerization of at least one polymerizable ethylenic monomer suspended in an aqueous medium often requires the presence of other materials, such as finely divided inorganic particles and other additives. For example, the process disclosed in U.S. Pat. No. 2,673,194 to Grim requires the presence of an anionic surface active agent and the process disclosed in U.S. Pat. No. 2,801,921 to Hutchinson et al. requires the presence of excess alkali and finely divided magnesium hydroxide. Accordingly, there was a need for an improved and more effective aqueous polymerization process for forming a suspension of distinct styrene butadiene copolymer particles which may readily be separated from the reaction medium by mere filtration. There also was a need for a polymerization process which would provide toner polymers possessing properties necessary to meet the demanding requirements of modern high speed electrostatographic imaging systems. These and other needs were provided with the process for the preparation of styrene butadiene copolymers as illustrated in U.S. Pat. No. 4,588,108, the disclosure of which is totally incorporated herein by reference. In one embodiment of the aforementioned patent, there is provided a process for forming discrete particles of a copolymer of styrene and butadiene in which a vapor phase and an aqueous phase comprising a mixture of water, styrene monomer, butadiene monomer, a suspension stabilizing agent, and a chain-propagating amount of a free radical polymerization initiator insoluble in water, soluble in the styrene monomer, soluble in the butadiene monomer and having a 1 hour half life between about 50.degree. C. and about 130.degree. C., the ratio of the styrene monomer and the butadiene monomer being between about 80:20 and about 95:5 by weight, the weight proportion of water to the combination of the styrene monomer and the butadiene monomer being between about 0.8:1 and about 2:1, the suspension stabilizing agent consisting essentially of finely divided, difficulty water soluble powder, is heated in an inert atmosphere to a temperature between about 50.degree. C. and about 130.degree. C. at a pressure between about 20 psi and about 140 psi in the absence of redox initiators and mercaptan compounds, removing butadiene monomer from the vapor phase after at least about 75 percent by weight of the butadiene monomer and styrene monomer in the aqueous phase are converted to a copolymer and prior to conversion of more than about 98 percent by weight of the butadiene monomer and styrene monomer in the aqueous phase to a copolymer and continuing heating in an inert atmosphere at a temperature between about 50.degree. C. and about 130.degree. C. at pressure between about 20 psi and about 140 psi until the Tg value of the discrete copolymer particles formed is between about 45.degree. C. and 65.degree. C. and the weight average molecular weight of the discrete copolymer particles is between about 10,000 and about 400,000. Increased molecular weight distribution of the final copolymer may be achieved by introducing an additional mixture of styrene monomer, butadiene monomer, suspension stabilizing agent and initiator to the aqueous mixture at least once during the heating step. The additional initiator is added in a different proportion relative to the new charge of monomer compared to the origin aqueous mixture. Optimum yields and minimum residual monomer content are achieved by heating the aqueous mixture with at least two different initiators in accordance with predetermined multistage heating procedures. Any suitable styrene monomer for polymerizable styrene derivative may be employed in the polymerization process of the aforementioned patent according to the teachings thereof. Typical polymerizable styrene derivatives disclosed include alpha-methylstyrene, vinyltoluene, ethylstyrene, monochlorostyrene, dichlorostyrene, alkoxystyrenes such as paramethoxystyrene and the like. Styrene is preferred because of its low cost and availability. The other monomeric reactant employed in the process of this the above patent is 1,3-butadiene. Also, according to the teachings of this patent any suitable chain-propagating amount of a free radical polymerization initiator insoluble in water, soluble in the styrene monomer and soluble in the butadiene monomer may be employed in the process of this invention. Typical monomer soluble free radical polymerization initiators include n-lauryl peroxide, benzoyl peroxide, acetyl peroxide, decanoylperoxide, azo-bis-isobutyronitrile t-butyl butylperbenzoate, O,O-t-butyl-O-(ethylhexyl)monoperoxycarbonate, peroxydicarbonates 2,2-azo-bis(2,4-dimethyl-l-4-methoxyvaleronitrile), 2,2-azo-bis(2,4-dimethylvaleronitrile), and mixtures thereof. Optimum results are achieved with peroxides, peroxycarbonates peroxybenzoates, azonitrile free radical polymerization initiators, and the like. These free radical polymerization initiators should possess a half life of about 1 hour at temperatures between about 50.degree. C. and about 130.degree. C. in order to effect adequate polymerization at reaction temperatures between about 50.degree. C. and about 130.degree. C. for reaction times of less than about 8 hours. Satisfactory results may be achieved when the reaction mixture comprises from about 0.05 percent to about 6.5 percent by weight of the free radical polymerization initiator based on a total weight of the styrene monomer and butadiene monomer. A range of about 0.05 percent to about 6 percent by weight of the free radical polymerization initiator is preferred because it provides an acceptable rate of polymerization and leads to the synthesis of copolymers with molecular properties which enable toners containing these copolymers to melt at low temperatures. Too high a concentration leads to too low a molecular weight. Reaction time is excessive when the concentration of initiator is less than about 0.05 percent. Moreover, the suspension can become unstable and result in polymers having unduly high molecular weight when the initiator concentration is too low.
Disadvantages associated with the process of the aforementioned '188 patent can include long reaction times, for example the reaction time from initiation to completion can be 362 minutes, which includes 45 minutes to heat the reactor to 95.degree. C. from ambient temperature, 192 minutes for the reaction to proceed at 95.degree. C., 40 minutes for the reaction temperature to be increased from 95.degree. C. to 125.degree. C., 60 minutes for the reaction to proceed at 125.degree. C. and 25 minutes for the reactor to be cooled to ambient temperature. These disadvantages can be avoided or minimized with the process of the present invention wherein there is selected as free radical polymerization initiator Lupersol TEAC, O,O-t-amyl-O-(2-ethylhexyl)monoperoxide carbonate. More specifically, with the aforementioned Lupersol available from Lucidol Division of Pennwalt Corporation the reaction time of the process as illustrated in the '188 patent can be reduced at least by one hour in an embodiment of the present invention, thereby enabling, for example, the reaction to be completed in 302 minutes rather than 362 minutes in an embodiment. Also, 33 minutes can be reduced from the part of the reaction where the monomers react at 95.degree. C., and 27 minutes can be reduced from the portion of the reaction where the monomers react at 125.degree. C. in embodiments of the present invention.
Moreover, toner and developer compositions, especially those containing charge enhancing additives, especially additives which impart a positive charge to the toner resin, are well known. Thus, for example, there is described in U.S. Pat. No. 3,893,935 the use of certain quaternary ammonium salts as charge control agents for electrostatic toner compositions. There is also described in U.S. Pat. No. 2,986,521 reversal developer compositions comprised of toner resin particles coated with finely divided colloidal silica. According to the disclosure of this patent, the development of images on negatively charged surfaces is accomplished by applying a developer composition having a positively charged triboelectric relationship with respect to the colloidal silica. Further, there is illustrated in U.S. Pat. No. 4,338,390, the disclosure of which is totally incorporated herein by reference, developer and toner compositions having incorporated therein as charge enhancing additives organic sulfate and sulfonate compositions; and in U.S. Pat. No. 4,298,672, the disclosure of which is totally incorporated herein by reference.
In a patentability search report letter the following U.S. Patents were listed: U.S. Pat. No. 4,558,108 mentioned herein; U.S. Pat. No. 4,777,230 relating to free radical polymerization of certain monomers, and wherein according to the abstract these polymers are produced by a solution polymerization with an initiating amount of tertiary alkyl hydroperoxide or its deriviatives, such as monoperoxycarbonates, see column 2 for example; note column 7 wherein the initiator can be O,O-t-amyl-O-(2-ethylhexyl)monoperoxycarbonate, and in column 8 wherein the initiator can be Luperol.RTM. TAEC, O,O-t-amyl-O-(2-ethylhexyl)monoperoxycarbonate, marketed by the Lucidol Division of Pennwalt Corporation; column 9, Lupersol.RTM. TAEC, and, for example, columns 13 and 14; U.S. Pat. No. 3,326,859 which discloses a polymerization method with peroxycarbonates, see for example column 2; U.S. Pat. No. 4,277,592, see column 3 wherein as an initiator there is selected bis(2-ethyl-hexyl)percarbonate; U.S. Pat. No. 4,526,726, see column 3, for example; U.S. Pat. No. 4,613,656, see the Abstract of the Disclosure for example; and as background interest U.S. Pat. No. 25,763, U.S. Pat. Nos. 2,370,588; 2,475,648 and 2,839,519.